A progressive-power lens is widely used in general because of an advantage that it is hardly detected from others as a spectacle for the aged in spite of a spectacle lens for presbyopia, an advantage that it allows a wearer to clearly look continuously from a far distance to a near distance without discontinuity, and so on. However, it is also widely known that the necessity of arrangement of a plurality of visual fields such as a field for looking far and a field for looking near, and further a field for looking at a distance intermediate therebetween, without a boundary line existing within a limited lens area, presents disadvantages specific to the progressive-power lens such that each visual field is not always sufficiently wide, and that there is a region mainly in a side visual field which causes the wearer to feel distortion or sway of an image.
Various proposals have been made since long ago to improve the disadvantages specific to the progressive-power lens, and most of such conventional progressive-power lenses have a surface configuration created by a combination of a “progressive surface” arranged on an object side surface and a “spherical surface” or a “cylindrical surface” arranged on an eyeball side surface. Conversely to those, Atoral-Variplas as a progressive-power lens, which is characterized in that a “progressive action” is added to the eyeball side surface, is released in 1970 from Essel Optical Co. (now Essilor), France.
Besides, recently proposed prior arts include, for example, technologies described in Patent International Publication Nos. WO 97/19382 and WO 97/19383 and so on, which are generally called rear surface progression (or concave surface progression). The surface configuration in the recently proposed rear surface progression has a main purpose of improving distortion and sway of an image by allotting a portion or the whole of a necessary addition diopter from an object side surface to an eyeball side surface to reduce the magnification difference of an image between a distance portion and a near portion.
Among these prior arts, one described in WO 97/19382 has a configuration in which the object side surface is made a spherical surface or a rotationally symmetrical aspherical surface to completely cancel the “progressive action,” and a “progressive surface” providing a predetermined addition diopter is added (fused) to only the eyeball side surface. Besides, the prior art described in WO 97/19383 has a configuration in which the addition diopter on the “progressive surface” being the object side surface is made lower than a predetermined value and a “progressive surface” providing a deficiency in addition diopter is added (fused) to a “spherical surface” or “cylindrical surface” on the rear surface side.
Although having different purposes and reasons, other prior arts of the progressive-power lens having description of technologies of adding the “progressive action” to the eyeball side surface, include, for example, ones described in Japanese Patent Publication No. Sho 47-23943, Japanese Patent Laid-Open No. Sho 57-10112, Japanese Patent Laid-Open No. Hei 10-206805, and Japanese Patent Laid-Open No. 2001-21846. In addition, prior arts in which the “progressive action” is provided to both surfaces of a lens, as in one described in the aforementioned WO 97/19383, include ones described in Japanese Patent Laid-Open No. 2000-338452 and Japanese Patent Laid-Open No. Hei 6-118353. Commonly, in these prior arts, front and rear two surfaces together provide a necessary addition diopter.
These prior arts have a main purpose of improving distortion and sway of an image by allotting a portion or the whole of a necessary addition diopter on an object side surface to an eyeball side surface to reduce magnification difference between a distance portion and a near portion. Clear description, however, on reasons of their improved effects can be rarely found, and only partial description thereof is found just in the aforementioned Patent International Publication No. WO 97/19383 (hereinafter, Prior art 1) or the like. Namely, Prior art 1 discloses the following calculation equations for a lens magnification SM shown in the equation (1) to the equation (3), the lens magnification SM is used as a basic evaluation parameter for lens design.
Namely, Prior art 1 includes the following description.
“The lens magnification SM is generally expressed by the following equation.
 SM=Mp×Ms  (1),
where Mp is called a power factor, and Ms is called a shape factor. When the distance from a vertex of an eyeball side surface (inside vertex) of a lens to an eyeball is an inter-vertex distance L, a refractive power at the inside vertex (inside vertex refractive power) is Po, a thickness at the center of the lens is t, a refractive index of the lens is n, and a base curve (refractive power) of the object side surface of the lens is Pb, Mp and Ms are expressed as follows.Mp=1/(1−L×Po)  (2)Ms=1/(1−(t×Pb)/n)  (3)It should be noted that for calculations of the equation (2) and the equation (3), dioptry (D) is used for the inside vertex refractive power Po and the base curve Pb, and meter (m) is used for the distance L and thickness t, respectively.”
Then, these calculation equations for the lens magnification SM are used to calculate a difference in magnification between a distance portion and a near portion. In Prior art 1, it is regarded that the distortion and sway of an image are improved because of a small magnification difference.
The study by the inventor of the invention shows that though some effects are recognized in the above-described Prior art 1 as compared to its prior art, the following points need to be discussed to design a lens with higher performance.    a. Basic evaluation parameters used in the above-described Prior art 1 include a parameter which should be essentially applied only to a portion near the center of a lens as is clear from the description of “the distance L from a vertex of an eyeball side surface of a lens to an eyeball” and “a thickness t at the center of the lens.” More specifically, in an example of Prior art 1, the basic evaluation parameter to be applied only to a distance portion near the center of the lens, is applied also to a near portion positioned at a great distance below the lens center, thus presenting a possibility of error.    b. In Prior art 1, the lens magnification SM is calculated using five basic evaluation parameters, composed of the aforementioned ones with addition of the “refractive index of the lens n.” However, as is instantly found when tilting forward and backward a lens having an actual diopter, it is considered that the size of an image is strongly influenced by an “angle between a sight line and a lens surface.” This leads to a consideration that the “angle between a sight line and a lens surface” is nonnegligible particularly in calculation of the magnification of the near portion positioned at a great distance below the lens center. Accordingly, the lens design of Prior art 1 has a possibility of error caused by the “calculation of the lens magnification without consideration of the angle between a sight line and a lens surface.”    c. Prior art 1 only describes the “magnification” for an application example to a cylindrical lens but lacks idea on direction thereof, which causes a possibility of error when “magnifications in the vertical direction and the horizontal direction are different” which occurs, for example, in the near portion positioned at a great distance below the lens center.    d. To accurately calculate the magnification for the near portion, the distance to a visual target, that is, an “object distance” should be added as a calculation factor. In Prior art 1, the “object distance” is not taken into consideration, which presents an undeniable possibility of error.    e. In the magnification calculations, the influence by a prism action is not taken into consideration, which may cause an error.
As described above, the prior art may not be always sufficient from a viewpoint, in particular, of more accurately calculating the “magnification.”
The present invention is made to solve the above problems, and its object is to provide a bi-aspherical type progressive-power lens which provides an excellent visual acuity correction for prescription values and a wide effective visual field with less distortion in wearing, by reducing a magnification difference of an image between a distance portion and a near portion through correct calculation of the magnification of the image with an influence by an “angle between a sight line and a lens surface” and an “object distance” taken into consideration.
It is another object of the present invention to provide a bi-aspherical type progressive-power lens which makes it possible to use a “bilaterally symmetrical semifinished product” as an object side surface and process after acceptance of an order only an eyeball side surface into a bilaterally asymmetrical curved surface coping with a convergence action of an eye in near vision, and to reduce processing time and cost.